MessageBox box = new MessageBox();
box.setButtons(MessageBox.YESNOCANCEL);
box.setIcon(MessageBox.QUESTION);
box.setTitle("Save Changes?");
box.addCallback(l);
box.setMessage("You are closing a tab that has unsaved changes. Would you like to save your changes?");
box.show();

If I remove line "box.setIcon(MessageBox.QUESTION); ", the window appears.